1. Field of the Invention
The present invention relates generally to improvements in oil and gas well cementing and more particularly, but not by way of limitation, to multiple stage cementing of oil and gas wells.
2. Description of the Prior Art
In preparing oil well bore holes for oil and/or gas production, a most important step involves the process of cementing. Basically, oil well cementing is the process of mixing a cement-water slurry and pumping it down through steel casing to critical points located in the annulus around the casing, in the open-hole below, or in fractured formations.
Cementing a well protects possible production zones behind the casing against salt water flow and protects the casing against corrosion from subsurface mineral waters and electrolysis from outside. Cementing also eliminates the danger of fresh drinking water and recreational water supply strata from being contaminated by oil or salt water flow through the borehole from formations containing those substances. It further prevents oil well blowouts and fires caused by high pressure gas zones behind the casing and prevents collapse of the casing from high external pressures which can build up underground.
A cementing operation for protection against the abovedescribed down-hole condition is called primary cementing. Secondary cementing includes the cementing processes used in a well during its protective life, such as remedial cementing and repairs to existing cemented areas. The present invention is generally useful in both primary and secondary or remedial cementing.
In the early days of oil field production, when wells were all relatively shallow, cementing was accomplished by flowing the cement slurry down the casing and back up the outside of the casing in the annulus between the casing and the borehole wall.
As wells were drilled deeper and deeper to locate petroleum reservoirs, it became difficult to successfully cement the entire well from the bottom of the casing and, therefore, multiple stage cementing was developed to allow the annulus to be cemented in separate stages, beginning at the bottom of the well and working upwardly.
Multiple stage cementing is achieved by placing cementing tools, which are primarily valve ports, in the casing or between joints of casing at one or more locations in the borehole; flowing cement through the bottom of the casing, up the annulus to the lowest cementing tool in the well; closing off the bottom and opening the cementing tool; and then flowing cement through the cementing tool up the annulus to the next upper stage, and repeating this process until all the stages of cementing are completed.
U.S. Pat. Nos. 3,524,503, 3,768,556 and 3,768,562, all to Eugene E. Baker and assigned to Halliburton Company, Duncan, Oklahoma, disclose three forms of cementing tools currently used in multi-stage cementing. These three patents are incorporated herein by reference. The employment of the cementing tools disclosed in U.S. Pat. Nos. 3,768,556 and 3,768,562 and other prior art multiple stage cementing tools is quite satisfactory for many multiple stage cementing applications.
There are, however, cementing applications which necessitate the sealing off of the annulus between the casing string and the wall of the borehole at one or more positions along the length of the casing string. An example of such an application is when it is desired to achieve cementing between a high pressure gas zone and a lost circulation zone penetrated by the borehole. Another application is when it is desired to achieve cementing above a lost circulation zone penetrated by the borehole. A third application occurs when formation pressure of an intermediate zone peentrated by the borehole is greater than the hydrostatic head of the cement to be placed in the annulus thereabove. Still another application occurs when a second stage of cement is to be placed at a distant point up the hole from the top of the first stage of cement and a packer is required to help support the cement column in the annulus. A last example of an application for employment of a cementing packer occurs when it is desired to achieve full hole cementing of slotted or perforated liners.
The prior art contains teachings of the employment of inflatable packers, such as that disclosed in U.S. Pat. No. 3,524,503, and compression type packers for isolating various zones in the annulus during a cementing operation. However, such packer apparatus are subject to unseating, if set with inadequate sealing force, when the weight of the cement column in the annulus thereabove becomes too great. Also, due to irregularities in the wall of the borehole often encountered at the point where the packer is to be applied, compression type packers are often incapable of achieving a sufficient seal between the casing string and the wall of the well bore to achieve satisfactory multiple stage cementing results. Operation of the inflatable packer of U.S. Pat. No. 3,524,503 requires the use of three plugs of progressively increasing diameter thereby limiting the number of cementing stages which may be performed on a casing string of a given diameter.
The present invention overcomes these difficulties by providing a cementing tool either requiring two plugs for operation or mechanically operated from the ground surface and inflatable in response to the application of fluid pressure downwardly through the casing string into the interior of the inflatable packer element to achieve a positive seal between the exterior of the casing and the wall of the well bore prior to the introduction of cement into the annulus above the inflated packer. The latter form of the invention provides means for opening and closing the cementing ports thereof an unlimited number of times during the cementing operation.